1. Field of the Invention
The present invention relates to a knock detecting apparatus for internal combustion engines in which the occurrence of knocking in an engine is detected so as to control the ignition timing optimally. More particularly, the invention relates to a knock detecting apparatus which detects the occurrence of knocking from the cooling water pulsations due to the fact that the pressure vibrations caused by the occurrence of knocking in the engine cylinder propagate into the cooling water through the cylinder wall.
2. Description of the Prior Art
It is well known in the art that there is a close correlation between the timing of ignition and cylinder pressure. When a mixture is exploded but no knock is present, there occurs no superposition on the cylinder pressure of any higher frequency components (usually the frequency components in the ranges of 5 to 10 kHz and 11 to 13 kHz which fall in the frequency bands each determined by the engine cylinder bore diameter and the velocity of sound in the combustion and which are produced by the intermittent and rapid combustion). Upon occurrence of knock, such higher frequencies become superimposed on the cylinder pressure at around the peak cylinder pressure value and this has the effect of producing vibrations or sound outside the cylinder. Examination of the pressure signals generated inside the cylinders and detected as vibrations or sound outside the cylinders shows that the occurrence of knock (trace knock) commences at an engine crank angle at which the cylinder pressure attains the peak value and that as the knock is increased gradually (to light knock and then to heavy knock) a phenomenon occurs in which the higher frequency components start to superpose considerably on the cylinder pressure earlier (or on the ignition side) than the peak cylinder pressure crank angle.
As a result, it is possible to determine the presence of knocking by detecting the presence of engine vibrations due to the higher frequency components. The vibrations due to knocking occur within the lower frequency range of 5 to 10 kHz and the higher frequency range of 11 to 13 kHz as mentioned previously. Thus, if a diaphragm which is resonant at the cooling water pressure pulsations within these frequency ranges is used to detect the water pressure pulsations, it is possible to detect the occurrence of knocking with a high degree of accuracy.
However, because these water pressure pulsations are usually lower than 0.1 kg/cm.sup.2 and it is the usual practice to apply a hydrostatic pressure of 0.9 to 1.5 kg/cm.sup.2 to the cooling water system, preferably the hydrostatic pressure in the cooling water system should be eliminated and only the pulsation components should be detected.